Roger Ebert, the popular film critic and television co-host who along with his fellow reviewer and sometime sparring partner Gene Siskel could lift or sink the fortunes of a movie with their trademark thumbs up or thumbs down, died on Thursday. He was 70.

His death was announced by The Chicago Sun-Times, where he had worked for many years.

Mr. Ebert’s struggle with cancer, starting in 2002, gave him an altogether different public image — as someone who refused to surrender to illness. Though he had operations for cancer of the thyroid, salivary glands, and chin, lost his ability to eat, drink, and speak (he was fed through a tube and a prosthesis partly obscured the loss of much of his chin) and became a gaunter version of his once-portly self, he continued to write reviews and commentary and published a cookbook on meals that could be made with a rice cooker.

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‘‘When I am writing, my problems become invisible, and I am the same person I always was,’’ he told Esquire magazine in 2010. ‘‘All is well. I am as I should be.’’

In recent years, Mr. Ebert had written extensively about his illness on Twitter.

It would not be a stretch to say that Mr. Ebert was the best-known film reviewer of his generation, and one of the most trusted. He liked to say his approach — dryly witty, occasionally sarcastic, sometimes quirky in his opinions — reflected the working newspaper reporter he had been, not a formal student of film. His tastes ran from the classics to boldly independent cinema to cartoons, and his put-downs could be withering.

‘‘I will one day be thin, but Vincent Gallo will always be the director of ‘The Brown Bunny,’ ’’ he wrote.

In 1975 he became the first film critic to win a Pulitzer Prize, for his Sun-Times reviews. His columns were syndicated to more than 200 newspapers in the United States and abroad, and he wrote more than 15 books, many by skillfully recycling his columns. In 2005, he became the first critic to be honored with a star on the Hollywood Walk of Fame.

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‘‘In the century or so that there has been such a thing as film criticism, no other critic has ever occupied the space held by Roger Ebert,’’ Mick LaSalle, film critic for The San Francisco Chronicle, wrote in 2010. ‘‘Others as influential as Ebert have not been as esteemed. Others as esteemed as Ebert have not had the same direct and widespread influence. And no one, but no one, has enjoyed the same fame.’’

With Siskel, Mr. Ebert essentially defined television film criticism. Their collaboration began in 1975, the year he won his Pulitzer. Mr. Ebert was asked to appear on WTTW, the public broadcasting station in Chicago, as co-host of a new movie-review program. He was intrigued, but then taken aback when told that Siskel, the film critic of The Chicago Tribune, would be his co-host.

“The answer was at the tip of my tongue: no,’’ Mr. Ebert told Time magazine in 1987.

As for Siskel, he said he initially had no desire to team up with ‘‘the most hated guy in my life.’’

But the pairing worked. The show, originally titled ‘‘Opening Soon at a Theater Near You,’’ was a public television hit. It evolved into ‘‘Sneak Previews,’’ which went national when the Public Broadcasting Service began syndicating it in 1978. It eventually attracted more viewers than any other entertainment series in the history of public television.

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Seeing its commercial potential, Tribune Entertainment acquired the show in 1982 and syndicated it under the title ‘‘At the Movies.’’ In 1986, Mr. Ebert and Siskel signed a contract with Buena Vista Television to syndicate the program under their own names. Most people knew the two as intellectually engaged, sweater-wearing, often contentious men sitting in cozy theater chairs ad-libbing about a film’s strengths and weaknesses. Mr. Ebert was the portly one with the owlish eyeglasses; Siskel the taller one who was losing his hair.

For all their combativeness, however, they actually agreed on a movie’s worth much more often than they differed.

‘‘We liked each other; we even loved each other,’’ Mr. Ebert told Television Week in 2005. ‘‘And we also had days when we hated each other.’’

They even hugged once, in 1985, when they appeared on ‘‘The Tonight Show’’ with Johnny Carson.

A typical Siskel and Ebert program reviewed five films. Either Mr. Ebert or Siskel would introduce a clip and then give his opinion. Then the other would weigh in. Their disagreements were more entertaining than their agreements, complete with knitted brows, are-you-serious head-shaking, and gentle (or not) barbs.

Siskel once taunted Mr. Ebert about his weight: ‘‘Has your application for a ZIP code come through yet?’’ Mr. Ebert came back with a dart about Siskel’s receding hairline: ‘‘The only things the astronauts saw from outer space were Three Mile Island and your forehead.’’

Finally, the denouement, harking back to the Roman Colosseum: both thumbs up, both down or, in a split decision, one of each. Mr. Ebert said that he was the one who had come up with the all-or-nothing gestures, and that Siskel had thought of trademarking them.

Siskel died of a brain tumor in 1999 at 53. Afterward, the show was renamed ‘‘Roger Ebert & the Movies’’ and began rotating co-hosts as a way of auditioning them. In September 2000 Richard Roeper, a fellow Sun-Times columnist, became the permanent co-host and the show was renamed ‘‘Ebert & Roeper.’’ Mr. Ebert left the show in 2006 because of his illness, and Roeper left in 2008.

Mr. Ebert believed a great film should seem new at every watching; he said he had seen ‘‘Citizen Kane,’’ his favorite, scores of times. His credo in judging a film’s value was a simple one: ‘‘Your intellect may be confused, but your emotions never lie to you.’’

Roger Joseph Ebert, an only child, was born June 18, 1942, in Urbana, Ill., to Walter Ebert and the former Annabel Stumm. The first movie he saw was the 1937 Marx Brothers comedy, ‘‘A Day at the Races,’’ at the Princess Theater in Urbana.

“It was part of a double feature shown with five cartoons, and you got 4½ hours of solid entertainment for exactly nine cents,’’ Mr. Ebert once recalled.

He was barely able to write when he started his journalistic career, publishing The Washington Street News in his basement and delivering copies to a dozen neighborhood houses. He worked at his grade school newspaper, edited his high school paper, and by age 15 was earning 75 cents an hour covering high school sports for The News Gazette in Champaign.

He went to many movies and published a science fiction magazine called Stymie. In 1958, he won a statewide speaking contest for simulating radio broadcasts. In 1964, Mr. Ebert graduated as a journalism major from the University of Illinois at Champaign-Urbana, where he was editor of The Daily Illini.

He did graduate study in English at the University of Cape Town under a Rotary International Fellowship. He then became a doctoral candidate in English at the University of Chicago but left to become a feature writer at the Sun-Times.

Though his knowledge of film was limited, he was named the paper’s first movie critic in 1967, when he was 24; newspapers at the time wanted younger film critics to speak to the young audiences attracted to movies such as ‘‘The Graduate’’ and ‘‘Bonnie and Clyde’’ as well as New Wave films by French directors like Francois Truffaut and Jean-Luc Godard.

Mr. Ebert got some firsthand moviemaking experience by writing a screenplay for the 1970 film ‘‘Beyond the Valley of the Dolls’’ for Russ Meyer, a director known for his campy B-flicks featuring busty women. Panned by fellow critics (“gratuitously violent,’’ Siskel said), the film seemed a point of pride for Mr. Ebert, who was paid $15,000 and never tired of talking about it. He wrote a half-dozen more screenplays for Meyer, one of which was produced: ‘‘Beneath the Valley of the Ultra-Vixens’’ (1979).

As a critic, Mr. Ebert quickly gained traction. In 1970, Time magazine called him ‘‘a cultural resource of the community.’’ In 1973, the Chicago Newspaper Guild cited him as ‘‘ushering in a new era of criticism in Chicago.’’

Mr. Ebert spoke out against the Motion Picture Association of America’s rating system, saying it lurched between being too restrictive and too lenient. He criticized Hollywood for not supporting documentaries and relying too much on digital effects and what he called gimmicks, like 3-D.

He revived his television career in January 2011 with a new film-review program on public television. Using a computer-generated facsimile of his own voice, he discussed classic, overlooked, and new films while co-hosts handled the ‘‘thumbs’’ judgments.

The thumbs-up-or-down approach drew scorn from some critics, who said it trivialized film criticism. Speaking to Playboy magazine in 1991, Mr. Ebert agreed that his program at the time was ‘‘not a high-level, in-depth film-criticism show.’’ But he argued that it showed younger viewers that one can bring standards of judgment to movies, that ‘‘it’s OK to have an opinion.’’

Mr. Ebert’s books included the essay collections ‘‘The Great Movies’’ and ‘‘The Great Movies II’’ and a collection of reviews titled ‘‘I Hated, Hated, Hated This Movie.’’ He also wrote a book about being a pedestrian in his favorite city: ‘‘The Perfect London Walk.’’

In July 1993 Mr. Ebert married Chaz Hammelsmith.

Since 1999, he had been host of ‘‘Ebertfest,’’ a film festival in Champaign, Ill. It is sometimes called Roger Ebert’s Overlooked Film Festival, or Roger Ebert’s Film Festival and was scheduled to run from April 17-21, the website said.

Mr. Ebert — who said he saw 500 films a year and reviewed half of them — was once asked what movie he thought was shown over and over again in heaven, and what snack would be free of charge and calorie-free there.